This invention relates in general to communication devices, and in particular, to communication devices susceptible to gain imbalance caused by mismatches and imperfections in circuitry that process in-phase and quadrature-phase signals.
Certain radio frequency (RF) applications perform signal processing operations by separating radio frequency signals into signal components representing in-phase and quadrature channels (I and Q channels). For example, in direct conversion receivers, in-phase and quadrature phase signals are utilized to help in demodulating received signals. Generally, signals on the I and Q channel have equal amplitudes and a ninety degree phase difference, and these signals are separately processed by similar circuitry that maintain the amplitude and phase relationships. However, imperfections and mismatches in implementing circuitry may lead to an amplitude or phase imbalance, i.e., a deviation from the ideal amplitude and phase relationships.
When phase or gain imbalance distorts the received signal, subsequent signal processing is impacted. The prior art has long used higher tolerance components in an attempt to avoid imbalances between the I and Q components. Such an approach has significant cost impact and may still not adequately address the problem. Another prior art approach attempts to account for imbalance by estimating and removing the imbalance. One such approach is described in U.S. Pat. No. 5,396,656 issued on Mar. 7, 1995, to Jasper et al., for a Method For Determining Desired Components Of Quadrature Modulated Signals. Here, a closed loop feedback technique is used to continuously determine an error signal by updating estimates of an imbalance component until the magnitude of the error signal is negligible. Yet another approach is described in U.S. Pat. No. 4,122,448 issued on Oct. 24, 1978, to Martin, for an Automatic Phase And Gain Balance Controller For A Baseband Processor. Martin uses a pilot signal to determine phase and amplitude imbalances, and these imbalances are corrected using a feedback circuit.
It is desired in the art to provide an effective solution to mitigate imbalance caused by imperfections or mismatches in separate circuitry that process I and Q signals. Preferably, such solution is easily adaptable to existing circuit designs and does not have substantial circuitry and/or signal processing requirements.